The ornamental design for a dustcap, as shown and described.
FIG. 1 is a perspective view of a dust cap showing my new design;
FIG. 2 is a front view of the same; the front and back being mirror images of one another;
FIG. 3 is a side view of the same, the left and right sides being mirror images of one another;
FIG. 4 is a top view of the same; and,
FIG. 5 is a perspective view of the dust cap, wherein the broken lines shown are included for the purpose of illustrating environmental structure that forms no part of the claimed design.